#define _CRT_SECURE_NO_WARNINGS 1
#include<iostream>

using namespace std;

int arr[26];

int main()
{
    string s;
    cin >> s;
    for (int i = 0; i < s.size(); i++)
    {
        arr[s[i] - 'a']++;
    }
    for (int i = 0; i < s.size(); i++)
    {
        if (arr[s[i] - 'a'] == 1)
        {
            cout << s[i] << endl;
            return 0;
        }
    }
    puts("no");
}
